1. Field of the Invention
The invention relates to a process for cleaning a semiconductor wafer using a cleaning solution, more precisely a process in which the semiconductor wafer is cleaned in an individual-wafer treatment mode.
2. Background Art
Individual-wafer cleaning processes have some advantages over multiwafer cleaning processes in which a plurality of wafers are cleaned simultaneously. These advantages include low consumption of chemicals, higher process flexibility and also the possibility of treating the wafer front side and the wafer rear side using different cleaning solutions. A particular advantage of individual-wafer treatment is that the risk of cross-contamination, that is to say the transfer of contaminants between semiconductor wafers, is precluded. The essential disadvantage of these processes in comparison with multiwafer treatment is the low throughput. Since silicon wafers having a very large diameter (300 mm) are increasingly being processed in the semiconductor industry, this disadvantage is receding into the background, however.
Individual-wafer cleaning can be effected according to two fundamentally different processes. Thus, the cleaning solution can be provided as a bath into which the semiconductor wafer is dipped for a certain time. More often, an alternative arrangement is used in which the semiconductor wafer is anchored on a rotating plate and the cleaning solution is injected or sprayed onto one or both lateral surfaces of the semiconductor wafer through nozzles. The spent cleaning solution flows away from the semiconductor wafer under the influence of centrifugal force. One example of such an individual-wafer treatment, which is also suitable for etching a semiconductor wafer, is described in US 2004/0031503 A1, for example.
In particular, semiconductor wafers on which one or a plurality of further layers are intended to be epitaxially deposited must have particularly intensively cleaned surfaces prior to deposition. Contaminants that must be removed are particle contaminants, metal ions bound on the surface, and organic compounds that often cover semiconductor wafers as a thin surface film. Basic solutions of ammonia and hydrogen peroxide in water (ammonium peroxide mixture, APM) are normally used for removing particles and organic compounds. A central prerequisite for the deposition of high-quality epitaxial layers is complete removal of the amorphous oxide layer coating the surface of semiconductor materials such as silicon, germanium and silicon/germanium alloys. The surface oxide is dissolved using aqueous solutions containing hydrogen fluoride, HF. A semiconductor wafer treated using an HF solution has a hydrophobic, oxygen free surface. A cleaning solution which is particularly suitable for this purpose on account of its high etching rate for silicon dioxide contains a mixture of ammonium fluoride, NH4F, or a tetraalkylammonium fluoride, e.g. tetramethylammonium fluoride N(CH3)4F, and hydrogen fluoride. WO 98/56726 A1 shows that it is advantageous for the cleaning solution not to be prepared until directly prior to use, from ammonium hydroxide or tetraalkylammonium hydroxide and hydrogen fluoride.